雅思阅读 87: The Otter That Keeps the Forest Under the Sea（守护海底森林的水獭）

改编自 U.S. Geological Survey / PNAS（2024年）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://pubs.usgs.gov/publication/70264274

Reading Passage

A. On a calm day, a kelp forest off the Pacific coast looks like a cathedral. Giant kelp, anchored by a small root-like holdfast, grows upward through cool water, its long fronds reaching toward the surface to form a floating canopy. Beneath that canopy, rockfish hover, octopuses hide in cracks, sea stars sprawl on the rocks and sea urchins graze. The forest teems with life. In the 1970s, the ecologist James Estes noticed something strange. Along some stretches of the Aleutian archipelago, the same rocky bottom was not a forest at all. It was what divers call an urchin barren: a flat, pale expanse of bare rock, paved with purple spines, with no kelp and almost nothing else. The difference between the two landscapes was not the temperature of the water, the depth or the type of rock. It was, quite simply, whether sea otters were present. That single observation, published with John Palmisano in 1974, became the founding example of an idea that reshaped ecology: the keystone species.

B. The logic of the otter effect is a food chain read from the top down. Sea urchins are slow-moving grazers that spend their lives on the seafloor, biting through the holdfasts of kelp with a hard beak. A single large urchin can defoliate a patch of kelp in weeks. Left unchecked, urchins spread across the bottom like a lawn mower gone feral, converting a three-dimensional forest into a flat, two-dimensional desert. Sea otters, however, love urchins. A fifty-pound otter must eat roughly a quarter of its body weight every day to keep warm in near-freezing water, and hard-shelled invertebrates — urchins, crabs, clams — are its staple. Where otters are abundant, urchins are suppressed, the ones that survive hide in crevices, and the kelp grows unmolested to the surface. The otter does not eat the kelp. It does not plant it. It merely keeps the urchins in check, and the forest returns by itself.

C. The word "keystone" comes from the wedge at the top of an arch, the one stone whose removal brings the whole structure down. The ecologist Robert Paine had coined it a few years earlier after removing starfish from a rocky shore and watching mussels overrun everything. Paine's experiment was brutal and simple: he pried the predatory starfish off a stretch of Pacific coast, threw them into the sea, and watched the food web underneath collapse. Within a few years, mussels had crowded out almost everything else, and the rich intertidal community he had been studying was a monoculture. The sea otter became the sea-floor version of Paine's starfish. Its impact on the community was far out of proportion to its biomass. Reintroduce it to a stretch of coast from which it had been hunted out, and within a decade the kelp canopy thickens, fish return, abalone and rockfish multiply, and the whole food web rewrites itself. Because kelp grows fast and fixes carbon rapidly, otter-rich forests also sequester carbon at rates estimated at many tonnes per hectare each year, a so-called blue-carbon service that policy-makers have begun to weigh in climate calculations. A predator, in other words, turned out to be a carbon-capture technology with fur.

D. The story, however, is not as simple as the textbook version suggests. Recent analyses by the U.S. Geological Survey, comparing thirty-year datasets from Vancouver Island in British Columbia and San Nicolas Island off the California coast, show that the otter's effect depends on the neighbourhood. At Vancouver Island, the classic cascade played out: otters returned, urchins crashed, kelp flourished. Around San Nicolas, the response was muted. Kelp increased only modestly, and the reason appears to be that sea urchins there already faced strong competition from other grazers, and that warm-water currents periodically favour urchins regardless of otter pressure. A 2024 review of trophic cascades worldwide reached a similar conclusion: the top-down effect of a predator is real, but it is shaped by bottom-up factors — water temperature, nutrient supply, the presence of other competitors — in ways that vary from place to place. The keystone, it turns out, fits the arch only if the rest of the stones happen to be arranged just so.

E. The longer history is darker. Sea otters were hunted almost to extinction in the maritime fur trade of the eighteenth and nineteenth centuries, reduced from perhaps hundreds of thousands of animals to a few thousand scattered survivors. Where they vanished, urchin barrens spread, and it is plausible — though debated — that the loss of otters contributed, through the disappearance of kelp-fed habitats, to the extinction of Steller's sea cow, a giant sea mammal last seen in 1768. Protection under international treaties in the twentieth century allowed otter populations to recover in patches, and reintroduction programmes — including the controversial and still incomplete effort on the Oregon coast — are attempting to stitch the range back together. The ecological argument for doing so is now well established; the social argument is harder, because fishing communities whose catches of urchins and abalone depend on exactly the animals the otter competes with are rarely enthusiastic. The otter, in the end, is not a symbol of balance so much as a reminder that balance is never neutral. When a single predator returns, some species win by a forest's width, and others lose by a fisherman's livelihood. Conservation policy on the Pacific coast now openly grapples with that trade-off: pay fishermen whose urchin beds the otters reclaim, or accept that the otter's return is, for some, an occupation rather than a restoration. The sea otter, in its stubborn appetite for sea urchins, has turned out to be a test case for a harder question than ecologists initially expected. The question is not whether ecosystems can recover. They can. It is whether human communities can be compensated, fairly and quickly enough, for the species that must give way when they do.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. How the food chain works from the top down ii. The meaning of the keystone concept and its wider value iii. Why sea otters were hunted for fur iv. A more complicated picture: context matters v. The dark history of near-extinction and what follows vi. The anatomy of a sea urchin's beak vii. How kelp forests reproduce sexually

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What is an "urchin barren"? A. A reef where sea urchins are absent. B. A seafloor stripped of kelp by unchecked urchin grazing. C. A forest where otters are overpopulated. D. A type of kelp that grows in cold water.

6. Why does the writer call the otter "a carbon-capture technology with fur"? A. Otters directly absorb carbon dioxide through their fur. B. Otter presence allows kelp forests to grow and sequester carbon. C. Otters produce methane that is captured by the forest. D. Otters build underwater structures from kelp.

7. What did the USGS comparison reveal about the otter effect? A. It is identical across every coastline studied. B. It is weaker around San Nicolas than at Vancouver Island due to local factors. C. It disappeared completely in both locations after thirty years. D. It depends on the number of starfish present.

8. Why are fishing communities often unenthusiastic about otter recovery? A. Otters prey directly on commercially caught fish. B. Otters compete with them for urchins and abalone. C. Otters drive away tourists from fishing towns. D. Otters carry diseases that affect fish stocks.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Estes and Palmisano first published their otter–urchin–kelp findings in 1974.
10. Sea otters eat roughly a quarter of their own body weight in food each day.
11. The reintroduction programme on the Oregon coast has now been fully completed.
12. Most modern ecologists now reject the idea of trophic cascades entirely.
13. Steller's sea cow was last seen in the eighteenth century.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Where otters are abundant, they keep sea (14) __________ in check, allowing giant kelp to form a forest that also acts as a (15) __________ carbon sink.

答案与解析

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